In the packaging field, containers made of biaxially-oriented or bioriented PET are now widely used for packaging carbonated and non-carbonated drinks, juices, and sauces. This is because biaxially-oriented PET has good mechanical strength, a good appearance, and high chemical inertia as regards the products in these containers. Moreover, it forms an effective barrier to the gases contained in the liquids and to the oxygen in the air, thus providing preservation of the products contained therein without oxidation.
It is known that containers made of biaxially-oriented PET obtained by the stretching and blowing of a preform raised to the PET biorientation temperature undergo considerable shrinkage when raised to a temperature greater than the glass transition temperature (Tg) of the PET, a phenomenon which makes them generally unsuited for filling with a hot substance.
This shrinkage results from the fact that the internal stresses created in the material during its biaxial-orientation (longitudinal stretching together with blowing, which causes a transversely-directed stretching) are released when the container is heated to a temperature greater than the glass transition temperature (Tg) of the material.
In this regard, it is well known that the thermal stability of containers made of biaxially-oriented PET produced by stretching-blowing is substantially increased by means of a thermal treatment, commonly known as thermofixation. In this procedure, a preform heated to a temperature suitable for biaxial-orientation is stretched bi-axially in a blowing mold so as to form an intermediate container.
Next, while this intermediate container is still in contact with the walls of the blowing mold, it is heated to a higher temperature for a certain period, thus causing it to undergo thermofixation.
Finally, the thermofixed container, which is kept under pressure so as to withstand temperature-induced shrinkage, is cooled to a temperature at which it preserves its shape when not under pressure. According to the many publications dealing with thermofixation, the temperatures suitable for the implementation of this thermal treatment generally range between 140.degree. C. and 250.degree. C.
Another procedure is known in which a preform, heated to a bi-axial orientation temperature, is blown in a hot mold and kept in contact with the walls of this mold. The temperature of the walls may be 40.degree. C. greater than the minimum orientation temperature. In a first embodiment, the molded container thus obtained is cooled to a moderate degree, by causing its temperature to fall by 10.degree. to 30.degree. C. following the introduction of a cooling fluid into the interior of the container. The cooled container is then removed from the blowing mold. In a second embodiment, the container thus formed is allowed to retract freely within the mold by effecting the partial or total decompression of the blowing fluid; the container is then blown once again in the same hot mold or in another cooled one. The container is then removed from the mold. In accordance with embodiments of this procedure, the stretching-blowing of the polyethylene terephthalate (PET) preforms occurs at 95.degree. in a hot mold whose temperature, kept continuously greater than the temperature of the preforms, ranges between 110.degree. and 140.degree. C. The contact times between the molded container and the walls of the hot mold are approximately 10 seconds.
These known procedures, however, give rise to certain disadvantages, such as the use of molds at relatively high temperatures (up to 250.degree. C.), the long periods during which the container is kept in the blowing mold, the use of several successive molds, and the use of costly cooling fluids. As a result, these procedures are difficult to implement on an industrial scale and lead to lower rates of container production.